1. Field of the Invention
This invention relates to a process for forming a damascene metal interconnect using chemical mechanical polishing, which is suitable for manufacturing a semiconductor device.
2. Description of the Prior Art
A damascene interconnect has been conventionally formed by forming a concave such as a groove or connection hole on an insulating film on a substrate, forming a barrier metal film on the insulating film, forming a conductive metal film over the whole surface such that the concave is filled with the metal, and then polishing the surface by chemical mechanical polishing (hereinafter, referred to as “CMP”). It will be described with reference to forming a damascene copper interconnect.
With regard to forming a semiconductor integrated circuit such as ULSI which has been significantly refined and compacted, copper has been expected to be a useful material for electric connection because of its good electromigration resistance and lower electrical resistance.
To date a copper interconnect is formed by a damascene interconnect forming process such as the above CMP due to problems such as difficulty in patterning by dry etching. Specifically, a concave such as a groove and a connection hole is formed in an insulating film, a barrier metal film is formed on the surface, a copper film is deposited over the whole surface by plating such that the concave is filled with the material, and then the surface is polished to be flat by CMP until the surface of the insulating film except the concave area is completely exposed, to form electric connections such as a damascene connection interconnect in which the concave is filled with copper, a via plug and a contact plug.
There will be described a process for forming a damascene copper interconnect with reference to FIG. 1.
As shown in FIG. 1(a), on a silicon substrate 1 are sequentially formed a silicon nitride film 2 and a silicon oxide film 3. On the silicon oxide film 3 is formed a concave having an interconnect pattern and reaching the silicon nitride film 2.
Then, as shown in FIG. 1(b), a barrier metal film 4 is formed by sputtering. On the film is formed a copper film 5 by plating such that the concave is filled with the material.
As shown in FIG. 1(c), the copper film 5 is polished by CMP to make the substrate surface flat. Polishing by CMP is continued until the metal over the silicon oxide film 3 is completely removed, as shown in FIG. 1(d).
In the above process for forming a damascene metal interconnect, a barrier metal film is formed as a base film for, e.g., preventing diffusion of the interconnect metal into the insulating film. However, when using a chemically quite stable metal including a tantalum-containing metal such as Ta and TaN as a barrier metal film, a polishing rate for the barrier metal film is smaller than that for the interconnect metal film in CMP using a conventional polishing slurry, i.e., there is a significant difference between the polishing rates for the barrier metal film and the interconnect metal film, which may cause dishing and erosion.
Dishing is a phenomenon that the interconnect metal in the concave is excessively polished so that the center of the interconnect metal film in the concave is depressed in relation to the plane of the insulating film on the substrate, as shown in FIG. 2. CMP using a conventional polishing slurry requires an adequately much polishing time for completely removing the barrier metal film 4 on the insulating film (silicon oxide film 3) because of a lower polishing rate for the barrier metal film. The polishing rate for the interconnect metal film (copper film 5) is higher than that for the barrier metal film 4, so that the interconnect metal film (copper film 5) is excessively polished, resulting in dishing.
Erosion is a phenomenon that polishing in a dense interconnect area excessively proceeds in relation to that in a sparse area such as an isolated interconnect area so that the surface of the dense interconnect area becomes depressed in relation to the other surfaces, as shown in FIG. 1(d). When the dense interconnect area comprising many damascenes of the interconnect metal film (copper film 5) is considerably separated from the isolated interconnect area comprising less damascenes of the interconnect metal film (copper film 5) by, for example, an area without interconnects within the wafer, and the interconnect metal film (copper film 5) is polished faster than the barrier metal film 4 or the insulating film (silicon oxide film 3), then a polishing pad pressure to the barrier metal film 4 or the insulating film (silicon oxide film 3) in the dense interconnect area becomes higher than that in the isolated interconnect area. As a result, in the CMP process after exposing the barrier metal film 4 (the process of FIG. 1(c) and thereafter), there generates a difference in a polishing rate between the dense interconnect area and the isolated interconnect area, so that the insulating film in the dense interconnect area is excessively polished, resulting in erosion.
Dishing in the process for forming a damascene interconnect in a semiconductor device as described above, may cause increase in an interconnection resistance and a contact resistance, and tends to cause electromigration, leading to poor reliability in the device. Erosion may adversely affect flatness in the substrate surface, which becomes more prominent in a multilayer structure, causing problems such as increase and dispersion in an interconnect resistance.
JP-A 8-83780 has described that dishing in a CMP process may be prevented by using a polishing slurry containing benzotriazole or its derivative and forming a protective film on a copper surface. JP-A 11-238709 has also described that a triazole compound is effective for preventing dishing. The technique, however, controls dishing by reducing a polishing rate for a copper film. Thus, a difference in a polishing rate between a copper film and a barrier metal film may be reduced, but polishing of the copper film takes a longer time, leading to a lower throughput. These publications have not discussed about erosion.
JP-A 10-44047 has described in its Examples that CMP may be conducted using a polishing slurry containing an alumina polishing material, ammonium persulfate (an oxidizing agent) and a particular carboxylic acid to increase a difference in a polishing rate between an aluminum layer for interconnection and a silicon oxide film and to increase a removal rate for a titanium film as a barrier metal film. The technique in the Examples cannot solve the above problems of dishing and erosion when using a chemically quite stable metal such as a tantalum-containing metal as a barrier metal film.
JP-A 10-46140 has described a polishing composition comprising a particular carboxylic acid, an oxidizing agent and water whose pH is adjusted by an alkali to 5 to 9. Examples in the publication have disclosed a polishing composition containing malic acid, citric acid, tartaric acid or oxalic acid as a carboxylic acid and aluminum oxide as a polishing material (Examples 1 to 4, 7, 8 and 11) and a polishing composition comprising malic acid as a carboxylic acid and silicon oxide as a polishing material (Example 12). However, this publication has described only improvement in a polishing rate and prevention of occurring dishing associated with a corrosion mark as an effect of addition of a carboxylic acid such as citric acid, and there are no descriptions for polishing a barrier metal film or erosion.
JP-A 10-163141 has disclosed a polishing composition for a copper film containing a polishing material and water, further comprising an iron (III) compound dissolved in the composition. Examples in the publication has described that a polishing rate for a copper film may be improved and surface defects such as dishing and scratches may be prevented, by using colloidal silica as a polishing material and iron (III) citrate, ammonium iron (III) citrate or ammonium iron (III) oxalate as an iron (III) compound. This publication has no descriptions about polishing for a barrier metal film consisting of a chemically quite stable metal such as a tantalum-containing metal or erosion.
JP-A 11-21546 has disclosed a slurry for chemical mechanical polishing comprising urea, a polishing material, an oxidizing agent, a film-forming agent and a complex-forming agent. Examples in this publication have described polishing Cu, Ta and PTEOS using a slurry having pH 7.5 prepared using alumina as a polishing material, hydrogen peroxide as an oxidizing agent, benzotriazole as a film-forming agent and tartaric acid or ammonium oxalate as a complex-forming agent. The publication, however, has described only that addition of the complex-forming agent such as tartaric acid and ammonium oxalate is effective for disturbing a passive layer formed by a film-forming agent such as benzotriazole and for limiting a depth of an oxidizing layer. It has described about Ta and TaN as examples for a barrier metal, but there are no descriptions about polishing for a barrier metal film consisting of a chemically quite stable metal such as a tantalum-containing metal or erosion.